After IC chips are formed on a semiconductor wafer (hereinafter, referred to as “wafer”), a probe test is performed on the wafer itself by using a probe apparatus in order to test electrical characteristics of the IC chips. The probe apparatus is constructed to control a position of a mounting table for mounting the wafer thereon capable of moving in X, Y and Z directions and rotating about a Z axis so that probes, e.g., probe needles, of a probe card are brought into contact with electrode pads of the IC chips of the wafer, the probe card being provided above the mounting table which mounts thereon the wafer.
A driving unit of the mounting table includes ball screws forming X, Y and Z axes and motors for driving the ball screws, and the position of the mounting table is controlled based on the number of pulses of an encoder attached to each motor. Therefore, the mounting table moves on coordinates of a driving system managed based on the number of pulses of the encoder. The contact position between the electrode pads of the wafer and the probe needles on the coordinates of the driving system can be calculated by photographing specific points on the wafer with a camera and obtaining a relationship among the position of the camera, the position of the mounting table in the photographing operation and the positions of the probe needles.
In practice, however, it is extremely difficult that the calculated contact position on the coordinates of the driving system coincide with the actual contact position by the effects of processing accuracy of the ball screws, distortion of the ball crews, right-left vibration or forward-backward inclination of a mechanism for guiding movement of the mounting table on an XY plane, inclination around peripheries of the ball screws, and further by the effects of extension and contraction of the ball screws and the like.
To that end, Patent Document 1 suggests a method for contacting probe needles with electrode pads by considering distortion of ball screws based on images of the probe needles captured by a camera provided at a mounting table and those of a wafer on the mounting table captured by a camera capable of moving inside an apparatus. In this method, positions of the probe needles and those of the electrode pads can be made effectively obtained as if they are recognized by a single camera by aligning optical axes of both cameras. The probe needles and the electrode pads are made to contact each other based on the imaging result and by considering the distortion of the ball screws and the like. Further, in this method, the alignment is carried out by considering a relationship between wafer coordinates (coordinates specified by arrangement of chips) and coordinates of the driving system by obtaining a relationship between the number of pulses of each encoder and the actual distance between two specific points respectively set in the horizontal and vertical directions in an area where chips are arranged. As a result, X, Y and Z coordinates of contact positions of points other than the specific points can be estimated.
Patent Document 2 describes a method for obtaining coordinate correction information indicating a relationship between ideal stage coordinates in an X-Y coordinate system which contain no misalignment and actually measured stage coordinates in an X-Y coordinate system which contain misalignment due to distortion of a stage at each of a plurality of reference points on a wafer that is mounted on the mounting table to be used in correcting coordinates. According to the entire disclosure of Patent Document 2, it may be possible that by employing this method, a contact position in the X and Y directions can be calculated with high accuracy by aligning the wafer to be inspected based on the coordinate correction information. Moreover, it may be also possible that in the method of Patent Document 1, X and Y coordinates of a contact position can be simply and precisely calculated based on such pre-obtained coordinate correction information. Further, in Patent Document 2, the Z direction is not considered at all.
FIG. 20A shows an ideal case that a surface of a wafer 900 is uniformly flat. In this case, if a Z position of a random point at which a wafer 900 contacts with a probe needle 904 is precisely obtained, the measurement can be stably performed even in other positions, i.e., electrode pads 901 and 902, by moving a mounting table 903 to the Z position. On the contrary, FIG. 20B describes a case that a surface of the mounting table is bent or a wafer surface seen from the probe needle is bent by the distortion of ball screws or the like. In such cases, a proper Z position (height position) at which the electrode pad 901 or 902 contacts with the wafer changes. Therefore, in order to stably measure all the IC chips on the wafer, the Z position needs to be properly set in each of the electrode pads.
Meanwhile, as an electrode pad becomes scaled-down, a dimension of about 40 μm×40 μm is being examined. The miniaturization of the electrode pad leads to a demand for higher accuracy in a height position at which the electrode pad and a needle tip contact with each other. Namely, after the wafer is aligned, the high accuracy of a Z coordinate as well as X and Y coordinates is required at a calculated contact position between probe needles and electrode pads. The reason is as follows.
After the probe needles and the electrode pads are brought into contact with each other, the mounting table is slightly raised. Accordingly, a so-called overdrive is applied, and the probe needles penetrate into the electrode pads. At this time, if the probe needles are horizontal probe needles, the probe needles are bent, and needle tips thereof are restored horizontally by the restoration force, thereby cutting a native oxide film on the surface of the electrode pads. Thus, if an overdrive is applied due to the error of the calculated coordinate of the contact position when the probe needles and the contact electrode pads are brought into contact with each other before the original overdrive is applied, the needle tips are already slightly bent at that moment. Accordingly, when the actual overdrive is applied at the position, the needle tips are deviated from the electrode pads.
Even for the case of vertical needles, a method is considered in which the needle tips thereof are made to slip when the overdrive is applied and the above problems are also encountered. Besides, the vertical needles require high accuracy of the Z coordinate of the original contact position regardless of whether the needles tips are made to slip or not. The reason is as follows. When an overdrive is already applied in the contact operation, the electrode pads are damaged by the needle tips when the intended over drive is applied. Accordingly, a large force is applied in a vertical direction to the vertical needles and, hence, the needles are bent, or the probe card becomes crooked. On the contrary, if the needle tips are slightly separated from the electrode pads in the contact operation, the intended overdrive is not applied sufficiently when actually applying the overdrive and, thus, the electrode pads are not damaged by the needle tips. For these reasons, an extremely high accuracy is required for fabricating the needle tips of the vertical needles. And thus, there arises a demand for easing such fabricating conditions.
The accuracy of the contact position in the Z direction cannot be guaranteed by the methods described in Patent Documents 1 and 2.
Patent Document 1: Japanese Patent Laid-open Application No. H7-297241
Patent Document 2: Japanese Patent No. 3725314: claim 1, FIGS. 3 to 5